A reciprocating engine requires a crankshaft to extract power by converting reciprocating motions of pistons in cylinders to rotational motion. There are two types of crankshafts: the type manufactured by die forging and the type manufactured by casting. Especially for multi-cylinder engines with two or more cylinders, the firstly mentioned die forged crankshafts that are excellent in strength and stiffness are often employed.
FIGS. 1 and 2 are side views schematically showing exemplary crankshafts for common multi-cylinder engines. The crankshafts 1 shown in FIGS. 1 and 2 are to be mounted in four-cylinder engines. Each of the crankshafts 1 includes five journals J1 to J5, four crank pins P1 to P4, a front part Fr, a flange Fl, and eight crank arms (which will hereinafter be referred to simply as “arms”) A1 to A8 that connect the journals J1 to J5 and the pins P1 to P4 to each other.
The crankshaft 1 shown in FIG. 1 includes counterweights W1 to W8 (which will hereinafter be referred to simply as “weights”) that are integrated with the eight arms A1 to A8, respectively. The crankshaft 1 is called a four-cylinder eight-counterweight crankshaft.
In the following paragraphs, when the journals J1 to J5, the pins P1 to P4, the arms A1 to A8, and the weights W1 to W8 are each collectively referred to, a reference character “J” is used for the journals, a reference character “P” for the pins, a reference character “A” for the arms, and a reference character “W” for the weights. A pin P and a couple of arms A (including weights W) connecting with the pin P are collectively referred to as a “throw”.
In the crankshaft 1 shown in FIG. 2, the front first arm A1, the rearmost eighth arm A8, and the two central arms (the fourth arm A4 and the fifth arm A5) have weights W integrated therewith. The other arms, namely, the second, the third, the sixth and the seventh arms A2, A3, A6 and A7 have no weights. This crankshaft 1 is called a four-cylinder four-counterweight crankshaft.
The journals J, the front part Fr and the flange Fl are arranged coaxially with the center of rotation of the crankshaft 1. Each of the pins P is located away from the center of rotation of the crankshaft by a distance corresponding to a half piston stroke. The journals J are supported by an engine block via slide bearings, and serve as a central axis of rotation. Each of the pins P is coupled to a big end of a connecting rod (which will hereinafter be referred to also as a “conrod”) via a slide bearing, and a piston is coupled to a small end of the conrod. The front part Fr is the front end of the crankshaft 1. A damper pulley 2 configured to drive a timing belt, a fan belt and so on is attached to the front part Fr. The flange Fl is the rear end of the crankshaft 1. A fly wheel 3 is attached to the flange Fl.
In an engine, fuel explodes inside the cylinders. The combustion pressures generated by the explosions act on the pins P of the crankshaft 1 via pistons and conrods. Meanwhile, the journals J are supported by bearings fixed to the engine block. Therefore, the arms A connecting the pins P and the journals J receive flexural loads and torsional loads. Then, the crankshaft 1 rotates while elastically deforming repeatedly in response to the different kinds of loads.
When the elastic deformation of the crankshaft 1 is great, the thicknesses of the oil films in the bearings supporting the journals J greatly change. This may cause galling in the bearings. Besides, the resistance to the rotations of the journals may increase, thereby causing a reduction in fuel economy performance. For this reason, it is preferred that the elastic deformation of the crankshaft 1 is small.
The amount of elastic deformation of the crankshaft 1 depends on the stiffness of the crankshaft 1 (especially the arms A). Since it is preferred that the elastic deformation is small, it is preferred that the stiffness is high. Usually however, an increase in the stiffness of the crankshaft 1 leads to an increase in the weight of the crankshaft 1.
Thus, there is a trade-off between increasing the stiffness of the arms A of the crankshaft 1 and reducing the weight of the crankshaft 1. In order to comply with both of the demands, various techniques relating to the arm shape have been suggested. There are conventional techniques as follows.
Japanese Patent Application Publication No. 2010-255834 (Patent Literature 1) discloses a crankshaft including an arm with a recess in the pin-facing surface. FIGS. 3A and 3B are schematic diagrams showing an arm shape similar to that disclosed in FIG. 3 of Patent Literature 1. FIG. 3A is a side view, and FIG. 3B is a front view from the pin side in the axial direction. The recess 10 extends across the entire width of the arm A.
Japanese Patent Application Publication No. 2000-320531 (Patent Literature 2) discloses a crankshaft including an arm with a recess and a reinforcing rib in the recess. The reinforcing rib is formed along a line Ac (which will hereinafter be referred to as an “arm centerline”) connecting the axis Jc of a journal J and the axis Pc of a pin P.